1. Field of the Invention
The present invention relates to a three-dimensional image display device and a three-dimensional image display method, where special eyeglasses are not used, particularly to a three-dimensional image display device and a three-dimensional image display method, where the visibility of a three-dimensional image is improved aiming at the reduction of viewer's fatigue.
2. Description of the Related Art
Conventionally, a display device capable of displaying three-dimensional images has been under study. Regarding binocular vision, Euclid who is a Greek mathematician considered in 280 B.C. that “Binocular vision is a sensation obtained when both the right and the left eyes simultaneously look at different images of a same object viewed from different directions” (refer to a literature “Three-dimensional display” written by Chihiro Masuda, published by Sangyo Tosho K.K., for example). Specifically, as a function of the three-dimensional image display device, it is necessary that images having parallax from each other be individually presented for both of the right and left eyes of the viewer.
Many three-dimensional image display methods are being studied as a method to specifically realize the function. The three-dimensional image display methods are largely divided into methods using eyeglasses and methods using no eyeglasses. Although the methods using eyeglasses are an anaglyph method using color difference, a polarized eyeglasses method using polarization, and the like, these methods essentially have to give viewers burdens of wearing eyeglasses, so that the study of the methods using no eyeglasses has been actively done in recent years.
The eyeglass-less methods are a lenticular lens method, a parallax barrier method, and the like. The lenticular lens method was invented by Ives, et al. around 1910 as described in the above-described literature. FIG. 1 is a perspective view showing the lenticular lens, and FIG. 2 is an optical model diagram showing a three-dimensional image display method using the lenticular lens. As shown in FIG. 1, one surface of the lenticular lens 21 is in flat surface and hog-backed convex portions (cylindrical lenses) 22 extending in one direction are formed in plural numbers on the other surface such that their longitudinal directions become parallel with each other.
Then, as shown in FIG. 2, a lenticular lens 21, a display panel 6, and a light source 8 are arranged sequentially from a viewer side, and the pixels of the display panel 6 are located on a focal plane of the lenticular lens 21. Pixels 23 displaying an image for the right eye 41 and pixels 24 displaying an image for the left eye 42 are alternately arrayed on the display panel 6. At this point, a group that consists of the pixels 23, 24 that are adjacent to each other corresponds to each convex portion 22 of the lenticular lens 21. Thus, light emitted from the light source 8 to transmit each pixel is distributed by the convex portions 22 of the lenticular lens 21 into directions for the right and left eyes. This allows the right and left eyes to recognize images that are different from each other, which enables the viewer to recognize the three-dimensional image.
On the other hand, Berthier invented the parallax barrier method in 1896, and Ives proved the idea in 1903. FIG. 3 is an optical model diagram showing the three-dimensional image display method using a parallax barrier. As shown in FIG. 3, a parallax barrier 5 is a barrier (light shield) on which a large number of thin vertically striped openings, that is, slits 5a are formed. And the display panel 6 is arranged near one surface of the parallax barrier 5. The pixels 23 for the right eye and the pixels 24 for the left eye are alternately arrayed on the display panel 6 in a direction orthogonal to the longitudinal direction of the slits. Further, the light source 8 is arranged near the other surface of the parallax barrier 5, that is, on the opposite side of the display panel 6.
The light, which has emitted from the light source 8, passed the openings (slits 5a) of the parallax barrier 5, and transmitted the pixels 23 for the right eye, becomes a light flux 81. In the same manner, the light, which has emitted from the light source 8, passed the slits 5a, and transmitted the pixels 24 for the left eye, becomes a light flux 82. At this point, a viewer's position from which the viewer can recognize a three-dimensional image is decided by a positional relation between the parallax barrier 5 and the pixels. Specifically, it is necessary that the right eye 41 of the viewer be within a passage region of all light fluxes 81 corresponding to a plurality of the pixels 23 for the right eye and the left eye 42 of the viewer be within the passage region of all light flux 82. This is a case where the midpoint 43 between the right eye 41 and the left eye 42 of the viewer positions in a square three-dimensional visible range 7 shown in FIG. 3.
Out of line segments in the three-dimensional visible range 7, which extend in the array direction of the pixels 23 for the right eye and the pixels 24 for the left eye, a line segment passing the intersection 7a of diagonal lines in the three-dimensional visible range 7 is the longest line segment. For this reason, when the midpoint 43 is located at the intersection 7a, latitude when the viewer's position shifts in either right or left direction becomes a maximum, and the position is most preferable as an observing position. Therefore, in the three-dimensional image display method, the distance between the intersection 7a and the display panel 6 is set as an optimal observation distance OD, and the viewers are recommended to view (observe) the image at this distance. Note that a virtual plane in the three-dimensional visible range 7, where the distance from the display panel 6 becomes the optimal observation distance OD is referred to as an optimal observation plane 7b. Thus, the light from the pixels 23 for the right eye and the pixels 24 for the left eye reaches the viewer's right eye 41 and left eye 42. Consequently, the viewer can recognize the image displayed on the display panel 6 as a three-dimensional image.
The parallax barrier method, when it was invented at first, had a problem that the parallax barrier had been an eyesore and caused low visibility because it was arranged between the pixels and the eyes. However, with the achievement of liquid crystal display devices in recent years, it has become possible to arrange the parallax barrier 5 on the rear side of the display panel 6 as shown in FIG. 3, and the visibility has been improved. Accordingly, the three-dimensional image display device of the parallax barrier method is actively studied.
A literature (Nikkei Electronics No. 838 issued on Jan. 6, 2003, pp. 26-27, Table 1) describes an example where a product has been commercialized using the parallax barrier method. This is a cellular phone mounting a 3D compatible liquid crystal display device, and the liquid crystal display device that constitutes the three-dimensional image display device has a size of diagonal 2.2 inches, which has the number of display dots of 176 dots in horizontal directions and 220 dots in vertical directions. Then, a liquid crystal panel that serves as the parallax barrier is provided, and turning the liquid crystal panel on/off allows it to display a three-dimensional image and a two-dimensional image. According to the product catalog and the user's manual of this product, the optimal observation distance in three-dimensional image display is 400 mm. In other words, a user can watch the three-dimensional image when observing it from a position 40 cm apart from the liquid crystal display device. Display definition of the conventional three-dimensional image display in two-dimensional image display is 128 dpi in both vertical and horizontal directions, but because an image for the left eye and an image for the right eye are displayed by arraying alternately in vertical striped shapes as described above during three-dimensional image display, the definition in the horizontal directions is 64 dpi that is half the definition in the vertical directions (128 dpi).
However, the above-described prior art has the following problems. Specifically, viewing three-dimensional images causes fatigue to the viewer's eyes and the like as described in the product catalog and the user's manual of the above-described conventional product. In other words, the viewer becomes tired by viewing the three-dimensional images for a long time.